[MSP430] Add support for .refsym directive

[krisb]

Summary

Introduces support for '.refsym' assembler directive.

From GCC docs:
'.refsym' - This directive instructs assembler to add an undefined reference
to the symbol following the directive. No relocation is created for this symbol;
it will exist purely for pulling in object files from archives.

lib/Target/MSP430/AsmParser/MSP430AsmParser.cpp

bool MatchAndEmitInstruction(SMLoc IDLoc, unsigned &Opcode,
                                bool MatchingInlineAsm) override;
 
   bool ParseRegister(unsigned &RegNo, SMLoc &StartLoc, SMLoc &EndLoc) override;
 
   bool ParseInstruction(ParseInstructionInfo &Info, StringRef Name,
                         SMLoc NameLoc, OperandVector &Operands) override;
 
   bool ParseDirective(AsmToken DirectiveID) override;
+  bool ParseDirectiveRefSym(AsmToken DirectiveID);
 
   unsigned validateTargetOperandClass(MCParsedAsmOperand &Op,
                                       unsigned Kind) override;
 
   bool parseJccInstruction(ParseInstructionInfo &Info, StringRef Name,
                            SMLoc NameLoc, OperandVector &Operands);
 
   bool ParseOperand(OperandVector &Operands);
if (getLexer().isNot(AsmToken::EndOfStatement)) {
     getParser().eatToEndOfStatement();
     return Error(Loc, "unexpected token");
   }
 
   getParser().Lex(); // Consume the EndOfStatement.
   return false;
 }
 
+bool MSP430AsmParser::ParseDirectiveRefSym(AsmToken DirectiveID) {
+    StringRef Name;
+    if (getParser().parseIdentifier(Name))
+      return TokError("expected identifier in directive");
+
+    MCSymbol *Sym = getContext().getOrCreateSymbol(Name);
+    getStreamer().EmitSymbolAttribute(Sym, MCSA_Global);
+    return false;
+}
+
 bool MSP430AsmParser::ParseDirective(AsmToken DirectiveID) {
   StringRef IDVal = DirectiveID.getIdentifier();
   if (IDVal.lower() == ".long") {
     ParseLiteralValues(4, DirectiveID.getLoc());
   } else if (IDVal.lower() == ".word" || IDVal.lower() == ".short") {
     ParseLiteralValues(2, DirectiveID.getLoc());
   } else if (IDVal.lower() == ".byte") {
     ParseLiteralValues(1, DirectiveID.getLoc());
+  } else if (IDVal.lower() == ".refsym") {
+    return ParseDirectiveRefSym(DirectiveID);
   }
   return true;
 }
 
 bool MSP430AsmParser::ParseOperand(OperandVector &Operands) {
   switch (getLexer().getKind()) {
     default: return true;
     case AsmToken::Identifier: {

test/MC/MSP430/refsym.s

+# RUN: llvm-mc -filetype=obj -triple=msp430 %s | llvm-readobj -t - | FileCheck %s
+
+foo:
+  .refsym __hook
+
+; CHECK:       Symbol {
+; CHECK:         Name: __hook (30)
+; CHECK-NEXT:    Value: 0x0
+; CHECK-NEXT:    Size: 0
+; CHECK-NEXT:    Binding: Global (0x1)
+; CHECK-NEXT:    Type: None (0x0)
+; CHECK-NEXT:    Other: 0
+; CHECK-NEXT:    Section: Undefined (0x0)
+; CHECK-NEXT:  }